The present invention relates to the field of data files, and more particularly, to image data files and related display methods.
Multi-layer (or multi-page) images can be thought of as a set of images, all typically but not necessarily the same size, which are combined for the purpose of displaying on an output display device. Thus, multi-layer refers to multiple images in a single file. Each image in the file may be referred to as a layer. Multi-layer images may be used for image editing, graphic design, and/or animation (such as animation in web pages on the Internet). As will be known to those having skill in the art, the Graphics Interchange Format (GIF) is widely used as a file format for animation. The Graphics Interchange Format is discussed, for example, in U.S. Pat. No. 6,714,202.
The GIF standard includes layers (or multiple images) which may be provided in sequence order. Each layer of a GIF file may be of different size and may be positioned using offset coordinates to improve storage efficiency in cases where only small areas contain changes from one layer to the next. The GIF standard defines a virtual screen upon which each layer is composed. The GIF standard generally uses a control block structure to indicate how the layers in the file are to be displayed. Each layer of the file format may be preceded by a control block, which may include: information about the location of the top left corner in the virtual screen, information on how long the layer should be displayed before proceeding to the next layer in the file; and whether the layer should be removed prior to display of a next layer in the file. This control block based structure may allow for relatively simple software implementation of the decoder. In fact little additional coding may be required to implement a GIF decoder capable of correctly displaying multi-layer animated GIF images.
The GIF standard is further discussed in the GIF specification entitled “GRAPHICS INTERCHANGE FORMAT (sm)”, Version 89a, Jul. 31, 1990, pages 1-34.